Method for producing block-shaped polymethacrylimide foamed materials

ABSTRACT

The invention relates to a method for producing block-shaped and plate-shaped polymethacrylimide foamed materials by copolymerizing methylacrylic acid and methylacryl nitrile, by subjecting the copolymerizate to a post-polymerization and cyclization in order to form polyimide, and by converting the same into a foamed material. The invention is characterized in that the copolymerization is carried out in the presence of a mixture containing at least three initiators with graduated half-life periods. The polymerization can be controlled especially well and yields polymerizate plates with thicknesses of up to 80 mm which can be easily foamed. The invention makes it possible to produce filling materials, especially plates and blocks containing electrically conductive particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing polymethacrylimidefoamed materials as well as blocks, plates and the like of suchpolymethacrylimide foamed materials as well as the shaped articlesobtained as intermediate products from the copolymer of methacrylic acidand methacrylonitrile.

2. Discussion of the Background

It is known how to produce polymethacrylimide foamed materials in theform of blocks. The first step is production of a preliminary product,which is already obtained in appropriate plate shape, from methacrylicacid and methacrylonitrile by copolymerization. The copolymer is thencyclized to the imide. An expanding agent present in the reactionmixture ensures appropriate foam formation when heated.

Heretofore, direct production of suitable polymer plates has beenpossible only up to a thickness of 30 mm. Since it is absolutelynecessary to ensure a regular and reliable polymerization sequence,cooling and heating must be applied alternately during polymerization,especially because the polymerization can no longer be controlled andirregular structures are formed if heating is too intensive. Thesealternating cooling and heating phases are naturally cumbersome, andalso entail high energy and water consumption.

In addition, polymer plates with thicknesses of only up to 30 mm sufferfrom numerous disadvantages. For example, if foamed blocks of relativelylarge thickness are needed, a plurality of foamed plates must becemented together. Moreover, very large amounts of waste are producedfrom the edge regions during manufacture.

In German Patent No. 1817156 there is already described a method inwhich foamable plastics are produced in plate form by polymerizingmixtures of methacrylonitrile and methacrylic acid between two glassplates sealed with a flexible cord. An expanding agent such as formamideor monoalkylformamide is already added to the starting mixture. Radicalsources are also added, in the form, for example, of a two-componentmixture of tert-butyl perpivalate and benzoyl peroxide.

The polymerization takes place at temperatures of, for example, 40, 45or 48° C., and it lasts about 15 to 40 hours. The product is thentempered at about 100° C. and subsequently heated to 170 to 300° C.Cyclization to imide and foam formation take place at the lattertemperatures.

It is difficult to ensure regular polymerization, because thetemperature can very easily exceed the specified temperature.Temperature fluctuations must therefore be controlled very accuratelyand compensated for by alternating cooling or heating phases.

Plates thicker than 30 mm cannot be produced by this method, sincetemperature elevations occur repeatedly and, because of the relativelypoor thermal conductivity of the polymerization mixture, the heatgenerated cannot be dissipated rapidly enough. Uncontrollable andunmanageable temperature elevations naturally lead to further increaseof radical formation, and so the reaction very easily runs out ofcontrol.

A method similar to that of German Patent No. 1817156 is described inEuropean Patent 0356714 A1. As the radical source there is used, forexample, azobisisobutyronitrile, and 0.1 to 10 wt % of electricallyconductive particles is added to the mixture to be polymerized.

The aforesaid problems also occur in this method, and the polymer platesobtained according to Example 1 of this European Patent Application havea thickness of only 25 mm.

Although numerous methods for producing foamed plates ofpolymethacrylimides are already known, a need still exists for improvedmethods of producing same and also for improved polymethacrylimideplates.

In particular, therefore, a need exists for thick foamed blocks andespecially for a method with which such blocks can be produced withouthaving to cement a plurality of blocks together.

SUMMARY OF THE INVENTION

The object of the invention is therefore to provide such blocks,especially in plate form, as well as a method for producing the same,with which a uniform block with thicknesses up to 80 mm and more can beproduced in one step, which permits a reliable and controlledpolymerization process and which operates with reduced water and energyconsumption and which yields blocks in which relatively little waste isproduced during manufacture. Another object of the invention is toprovide an appropriate method in which the tempering bath, especiallythe tempering water bath, can be operated largely with constanttemperature.

This object is achieved by a method for producing block-shapedpolymethacrylimide foamed materials by copolymerization of methacrylicacid and methacrylonitrile as well as further copolymerizable monomersif necessary in the presence of radical-forming initiators,postpolymerization and cyclization of the copolymer to polyimide andtransformation to a foamed material, which is characterized in that thecopolymerization is performed in the presence of a mixture comprising atleast three initiators with graduated half lives.

The subject matter is therefore a method according to the teaching ofclaim 1. Further advantageous embodiments are described in claims 2 to7, and correspondingly produced blocks are described in claims 8 to 9.Further subject matter of the invention is methods according to claim10, polymer plates according to claim 11 and the use according to claim12.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the temperature dependence of the half-life of initiatorslisted in Table 2.

FIG. 2 shows the temperature dependence of the half-life of initiatorslisted in Table 2.

FIG. 3 shows the temperature dependence of half-lives of azo compoundslisted in Table 3.

FIG. 4 shows the temperature dependence of half-lives of azo compoundslisted in Table 3.

DETAILED DESCRIPTION OF THE INVENTION

In connection with the invention, graduated half lives means that the atleast three initiators each have different half lives at a giventemperature or have the same half life, but in different temperatureranges. Preferably there are used initiators which each have a half lifeof one half hour in temperature ranges that are at least 10° C. apart.

In a particularly advantageous embodiment of the inventive method, thereis used an initiator mixture which comprises agents that decompose atlow temperature, medium temperature and high temperature and that havehalf lives of 1 hour in the ranges of 40 to 80, 80 to 110 and 110 to200° C., preferably 110 to 150° C.

Preferably there are used initiator mixtures which comprise at leastfour initiators, each with a half life on the order of one hour indifferent temperature ranges.

The proportion of initiator mixture used can be varied within relativelybroad limits; in this way the polymerization time can be controlled andalso the polymerization temperature can be influenced by the proportionof initiators used. The quantities used in connection with the inventionare expressed as weight parts of initiator per 100 weight parts ofmonomer. It is advantageous to use a total proportion of initiatormixture ranging from about 0.2 to 0.3 weight parts per 100 weight partsof monomer, preferably 0.21 to 0.24 weight parts.

The weight ratio of the individual initiators relative to each other inthe initiator mixture can also be varied within relatively broad limits;preferably the weight ratio of the individual initiators relative toeach other ranges from 1:1 to 1:10, preferably 1:1 to 1:4. Suitableproportions and mixing ratios can be determined by simple preliminaryexperiments.

Further subject matter of the invention are foamed blocks, especiallyfoamed plates with a thickness of up to 80 mm, which are obtainableaccording to one of the foregoing methods.

In the first step of the production of block-shaped foamed materials,there are prepared monomer mixtures containing methacrylic acid andmethacrylonitrile as main constituents, preferably in a mole ratio ofbetween 2:3 and 3:2. In addition, further conomomers can be used, suchas esters of acrylic or methacrylic acid, styrene, maleic acid oritaconic acid, etc., the anhydrides thereof, vinylpyrrolidone, etc. Theproportion of such comonomers should be no more than 30 wt %, preferablyno more than 10 wt % of the two main constituents. Small proportions ofcross-linking monomers such as allyl acrylate can also be used.Preferably, however, the proportions should be at most 0.05 to 1 wt %.The polymerization mixture also contains at least three initiators, eachof which has a half life on the order of one hour in differenttemperature ranges. It is advantageous to use appropriate initiatorsfrom at least four temperature ranges. Examples of suitable temperatureranges are 50 to 57° C. for the first range, 60 to 80° C. for the secondrange, 85 to 105° C. for the third range and 115 to 125° C. for thefourth range. As initiator in each of the individual temperature ranges,it is possible to use a single compound as initiator, but it is alsopossible to use two or more initiators with the appropriate half livesin the corresponding temperature ranges.

As suitable initiators for use according to the invention in a mixturecomprising at least three initiators, there can be used standardinitiators such as are used for radical generation in radical-initiatedpolymerization. These include compounds such as organic peroxides, suchas dicumyl peroxide, peroxydicarbonates, such as diisopropylperoxydicarbonate, peresters, such as tert-butylperoxy 2-ethylhexanoateand the like. Further compound types that can form radicals are alsosuitable in connection with the invention. They include in particularazo compounds such as azobis(isobutyronitrile) andazobis(2,4-dimethylvaleronitrile).

Initiator mixtures that are particularly suitable in connection with theinvention are those whose components are selected from the followinginitiators: azobis(isobutyronitrile), t-butyl peroctoate, t-butylperbenzoate, t-butyl perpivalate, azobis(2,4-dimethylvaleronitrile),t-butyl perneodecanoate, dibenzoyl peroxide, bis(4-t-butylcyclohexyl)peroxydicarbonate, cumylperoxy neodecanoate,1,4-di(2-neodecanoylperoxyisopropyl)benzene and the like.

Table 1 lists suitable initiators, together with the temperatures forhalf lives of 10 hours, 1 hour and 1 minute, as well as the half livesin hours for a series of temperatures. Table 2 shows initiators who'sdependence of half life on temperature is illustrated in FIGS. 1 and 2.It is a simple matter to compose initiator mixtures by means of thegraphs of FIGS. 1 and 2.

Table 3 lists azo compounds which are suitable as initiators inconnection with the invention.

FIGS. 3 and 4 graphically illustrate half lives of several azo compoundslisted in Table 3.

The half lives of the usable initiators generally already are given bythe manufacturer. They can easily be determined analytically, for whichpurpose benzene has generally proved suitable as the solvent. Thedetermination is generally performed with 0.1 molar solution.

The mixture for copolymerization also contains expanding agents, whicheither decompose or evaporate at temperatures of about 150 to 250° C.,and in the process form a gas phase. Examples of nitrogen-containingcompounds for use as expanding agents are urea, monomethylurea orN,N′-dimethylurea, formamide or monomethylformamide. Furthernitrogen-free expanding agents are formic acid, water or monohydricaliphatic alcohols, especially those with three to eight carbon atoms.The expanding agents are generally used in proportions of 0.5 to 8 wt %relative to the monomers used.

Polymerization is expediently carried out in block form. During theproduction of blocks, for example in flat layers with thicknesses of upto 80 mm, the monomer mixture is disposed between two glass plates,which are sealed at their respective edges and form a kind of flatchamber. This flat chamber is surrounded by a water bath adjusted to thedesired polymerization temperature.

The polymerization can be achieved largely or over broad ranges underisothermal conditions, or in other words at constant water-bathtemperature. In many cases it is possible to keep the water-bathtemperature constant from the beginning to the end of polymerization. Ifnecessary, however, the water-bath temperature can also be kept constantfor a long time at first and raised after a specified time, in order toperform part of the polymerization at a higher temperature.

The water-bath temperature can also be kept constant in this nextpolymerization phase, which is performed at a higher temperature.

The chosen water-bath temperature depends on the thickness of thepolymerization chamber and on the formula used for polymerization. Inthis connection it is generally advantageous to shift the polymerizationtemperature and thus also the water-bath temperature to lower valueswith increasing density of the plate to be produced.

The appropriate temperature for the formula and thickness can beoptimized in each case by simple preliminary experiments.

It is self-evident that the temperature must be adjusted to the chamberthickness and the formula such that the heat liberated duringpolymerization can be adequately dissipated without allowing undesiredtemperatures to develop in the polymerization mixture duringpolymerization. After completion of the polymerization process, which iscontrolled by the surrounding water bath, postpolymerization isperformed in an oven. Postpolymerization is generally performed attemperatures of 38 to 140° C. In general, 10 to 1000 hours is sufficientfor final polymerization in the tempering oven.

After completion of polymerization, the block is heated to a temperatureof about 180 to 250° C., at which temperature cyclization to the imidestructure and foam formation take place. Times of 3 to 5 hours areusually sufficient for this thermal posttreatment.

The blocks obtained in the process are characterized by a homogeneousregular structure.

Further subject matter of the invention is a method for producingpolymer plates by copolymerization of methacrylic acid andmethacrylonitrile as well as further copolymerizable monomers ifnecessary in the presence of radical-forming initiators andpostpolymerization, characterized in that the copolymerization isperformed in the presence of a mixture comprising at least threeinitiators with graduated half lives.

These polymer plates are formed as intermediate products duringproduction of the block-shaped methacrylimide foamed materials accordingto the invention. Further subject matter of the invention is polymerplates obtainable by the method specified hereinabove.

It was particularly surprising that, with the inventive method, it ispossible to perform the polymerization isothermally, or in other wordsat constant temperature, and thus obtain polymer blocks which havethicknesses of up to 80 mm and which exhibit a uniform propertiesprofile through the entire thickness of the block, thus indicatingregular homogeneous polymerization. The foamed blocks produced therefromcan be manufactured as such, so that only very little waste is producedat the edge regions. In this way it is possible to produce shapedarticles of appropriate thickness using only a single block, whereas aplurality of blocks cemented together was formerly necessary for thispurpose. The resulting blocks or plates, which are foamed in one piece,preferably have thicknesses of 80 to 300 mm.

The foamed plates or foamed blocks according to the invention are veryvaluable materials and can be used in particular as components inaircraft construction. The polymer blocks produced according to theinvention are characterized by extremely regular structure, can bestored indefinitely and, if necessary, can be transformed to foamedplates from case to case by appropriate heating.

EXAMPLES

The invention will be explained in more detail by means of the followingexamples:

Example 1

A mixture of 61 parts of methacrylic acid, 39 parts ofmethacrylonitrile, 4.7 parts of formamide and 4.2 parts of 2-propanoland an initiator mixture of 0.3 parts of t-butyl perpivalate, 0.04 partsof t-butyl per-2-ethylhexanoate, 0.07 parts of t-butyl perbenzoate and0.077 parts of cumyl perneodecanoate is polymerized for 66 hours at awater-bath temperature of 38° C. between two glass plates spaced 23 mmapart and sealed by a sealing cord, after which postpolymerization isperformed for 24 hours in an oven at a temperature of 115° C. Regularplates are obtained. During the polymerization, which is controlled bythe water bath, temperature fluctuations are practically zero or onlyslight, and do not necessitate readjustment of the water-bathtemperature.

The obtained plates can be transformed without problems to foamed platesby heat treatment at a temperature of 170 to 200° C.

Example 2

A mixture identical to that of Example 1 is polymerized in a flatchamber whose glass plates are spaced 30 mm apart. In this case thewater bath is kept at 33° C. for 92 hours, at 33 to 38° C. for 10 hoursand at 38° C. for 14 hours. Tempering in the oven at temperatures of115° C. is performed for 40 hours.

The plates have uniform structure and can be transformed withoutproblems to foamed materials.

Example 3

A composition identical to that of Example 1 is polymerized in a flatchamber whose glass plates are spaced 50 mm apart. The water bath iskept at 28° C. for 198 hours, at 28 to 37° C. for 30 hours and at 37° C.for 24 hours. Postpolymerization at a temperature of 115° C. isperformed for 50 hours.

Examples 4 to 6

In the following examples, a polymerization mixture with the followingcomposition is polymerized: 56 parts of methacrylic acid, 44.0 parts ofmethacrylonitrile, 1.0 parts of formamide, 3.3 parts of 2-propanol, 0.31parts of allyl methacrylate, 0.03 parts of t-butyl perpivalate, 0.03parts of t-butyl per-2-ethylhexanoate, 0.1 parts of t-butyl perbenzoateand 0.077 parts of cumyl perneodecanoate. The polymerization isperformed in flat chambers whose glass plates are spaced 23 mm, 30 mmand 50 mm apart respectively. For 23 mm the polymerization time is 80hours at 37° C. and the postpolymerization time in the oven is 32 hours;for 30 mm spacing, 114 hours at 32° C., 10 hours at 32 to 38° C. and 24hours at 38° C. as well as postpolymerization in the oven for 45 hours;for 50 mm spacing the polymerization time is 198 hours at 27° C., 24hours at 27 to 37° C. and 24 hours at 37° C. Postpolymerization in theoven lasts for 57 hours. All plates exhibit regular structure and can beprocessed to foamed materials without difficulty.

Further examples are listed in tabular form in Table A.

TABLE A Polymerization Thickness temperature ΔT Initiators (mm) (° C.)(° C.) DIPND:V77:V69:V73 0.083:0.03:0.04:0.07 23 38 2.00.078:0.03:0.04:0.07 23 38 1.4 0.09:0.03:0.04:0.07 23 38 2.30.09:0.03:0.04:0.07 30 35 3.4 0.09:0.03:0.04:0.07 50 29 2.3

ΔT denotes the maximum temperature difference between water bath andpolymerization mixture during polymerization.

DIPND=1,4-di(2-neodecanoylperoxyisopropyl)benzene

TBPP=t-butyl perpivalate

TBPEH=t-butyl peroctoate

TBPB=t-butyl perbenzoate

The polymer plates obtained in this way have a uniform regularstructure. During polymerization, which is controlled by a constantwater-bath temperature (polymerization temperature), the temperature inthe polymerizing mass also remains practically constant. The differencesinside the polymerizing mass are only a few degrees, at most 3.4° C. Thetemperature during polymerization is monitored by a thermocoupleinserted into the polymerizing mass. The slight temperature fluctuationsare negligible, and are far removed from the temperature excursions thatoccur when the operation is performed with only one or two initiatorsand it is tried to control the reaction with an isothermal water bath.Under those conditions temperature excursions of 13° C. and more occur,whereby the polymerization becomes uncontrollable, acceleratedpolymerization occurs and plates are obtained which either are of lowquality or must be scrapped.

It was further surprising that the inventive process can also be appliedto the production of polymer plates or to the corresponding plate-shapedor sheet-shaped foamed materials that contain fillers. For example,plates containing standard fillers can be obtained. In a preferredembodiment of the invention, it is also possible to produce foamedmaterials which contain electrically conductive particles, especiallycarbon particles such as conductive carbon black, an example beingKetjenblack 600EC, a commercial product manufactured by Akzo NobelChemie. Besides carbon black, it is also possible to use carbon fibersas fillers either alone or in addition to other fillers.

Example 7

Production of polymer plates filled with carbon black

1680.00 g of carbon black (Ketjenblack 600EC) is dispersed for 50minutes in 41901.18 g of methacrylic acid, 32656.08 g ofmethacrylonitrile, 2982.29 g of 2-propanol, 149.11 g of MgO and 1.4911 gof allyl methacrylate. The dissolved initiators are then stirred in andthe polymerization solution is evacuated for 30 minutes. Thepolymerization solution is also stabilized with 50 ppm of quinone.

A second chamber is filled in the same way, except that dispersion lastsfor 45 minutes and evacuation for 40 minutes. A third chamber is filledwith the same composition, but dispersion is performed under vacuum for25 minutes, after which evacuation is continued for a further 20minutes. The polymerization was carried out at 34° C. for 73.25 hours inall three cases. Thereafter tempering was performed for 13 hours at 34to 60° C., 3 hours at 60° C., 10 hours at 60 to 100° C., 5 hours at 100to 115° C. and 3 hours at 115° C. The obtained polymer plates haveflawless quality. The temperature fluctuations during polymerization arenegligible.

TABLE 1 Half lives of organic peroxides 0.1 molar in benzene (unlessotherwise indicated) Activation Temperature [° C.] for INTEROX energy ahalf life of No. Code Chemical name [kJ/mol] 10 h 1 h 1 min⁵⁾ 1 ASCPACETYL CYCLOHEXANE SULFONYL PEROXIDE 124 31 46 75 2 DIPND1,4-DI(2-NEODECANOYLPEROXYISOPROPYL)BENZENE¹⁾ 114 37 54 85 3 CUPNDCUMYLPEROXY NEODECANOATE 115 38 55 90 4 ^(*)) PEROXYDICARBONATE 124 4157 90 5 IPPC DIISOPROPYLPEROXY DICARBONATE 119 44 61 95 6 TAPNDTert-AMYLPEROXY NEODECANOATE 113 44 62 100 7 TBPND Tert-BUTYLPEROXYNEODECANOATE 121 47 64 100 8 TAPPI Tert-AMYLPEROXY PIVALATE 121 53 71110 9 DCLBP DI(2,4-DICHLOROBENZOYL) PEROXIDE 121 54 72 110 10 TBPPITert-BUTYLPEROXY PIVALATE 121 56 74 110 11 INPDI(3,5,5-TRIMETHYLHEXANOYL) PEROXIDE 117 59 78 120 12 DP DIDECANOYLPEROXIDE 126 62 80 120 13 LP DILAUROYL PEROXIDE 126 62 80 120 14DI(2-METHYLBENZOYL) PEROXIDE 119 62 81 120 15 DHPEH2,5-DIMETHYL-2,5-DI(2-ETHYLHEXANOYLPEROXY)HEXANE 137 67 84 125 16 PMBPDI(4-METHYLBENZOYL) PEROXIDE 125 70 89 130 17 BP DIBENZOYL PEROXIDE 12672 91 130 18 TAPEH Tert-AMYLPEROXY 2-ETHYLHEXANOATE 126 72 91 130 19TBPEH Tert-BUTYLPEROXY 2-ETHYLHEXANOATE 135 74 92 130 20Tert-BUTYLPEROXY ISOBUTYRATE 130 77 96 135 21 TBPM Tert-BUTYLMONOPEROXYMALEATE 116 82 104 150 22 TMCH1,1-DI(tert-BUTYLPEROXY)-3,3,5-TRIMETHYLCYCLOHEXANE²⁾ 143 95 114 155 23CH 1,1-DI(tert-BUTYLPEROXY)CYCLOHEXANE²⁾ 138 97 117 160 24Tert-BUTYLPEROXY ISOPROPYLCARBONATE 138 97 117 160 25 TBPINTert-BUTYLPEROXY 3,5,5-TRIMETHYLHEXANOATE 147 100 119 160 26 DHPBZ2,5-DIMETHYL-2,5-DI(BENZOYLPEROXY)HEXANE 147 100 119 160 27 TBP-EHCTert-BUTYLPEROXY (2-ETHYLHEXYL)CARBONATE²⁾ 128 100 122 175 28Tert-BUTYLPEROXY ACETATE 149 102 121 160 29 TABP Tert-AMYLPEROXYBENZOATE²⁾ 143 102 122 160 30 TBPB Tert-BUTYLPEROXY BENZOATE 143 104 124165 31 BU 2,2-DI(tert-BUTYLPEROXY)BUTANE²⁾ 143 104 124 165 32 NBVn-BUTYL-4,4-DI(tert-BUTYLPEROXY) VALERATE 141 110 131 175 33 EBUETHYL-3,3-DI(tert-BUTYLPEROXY) BUTYRATE³⁾ 144 114 135 180 34 DCUPDICUMYL PEROXIDE 152 116 136 175 35 BCUP Tert-BUTYLCUMYL PEROXIDE 154118 138 180 36 DTAP DI(tert-AMYL) PEROXIDE⁴⁾ 129 118 142 190 37 DIPPDI(tert-BUTYLPEROXYISOPROPYL)BENZENE 142 120 142 190 38 DHBP2,5-DIMETHYL-2,5-DI(tert-BUTYLPEROXY)HEXANE 142 120 142 190 39 DTBPDI(tert-BUTYL) PEROXIDE 152 125 146 190 40 DYBP2,5-DIMETHYL-2,5-DI(tert-BUTYLPEROXY)HEX-3-YNE 154 128 149 195 41 HMCN3,3,6,6,9,9-HEXAMETHYL-1,2,4,5-TETRAOXACYCLONONANE²⁾ 146 135 158 205 42TBHP Tert-BUTYL HYDROPEROXIDE 149 173 200 260 43 CC DFH3,4-DIMETHYL-3,4-DIPHENYLHEXANE 150 182 210 270 44 CC DFB2,3-DIMETHYL-2,3-DIPHENYLBUTANE 195 210 234 285 (Half lives in hours)No. Nr. 20° 30° 40° 50° 60° 70° 80° 90° 100° 110° 120° 130° 140° 150°160° 170° 180° 190° 1 57 11 2.4 0.5 — — — — — — — — — — — — — 2 — 40 7.31.9 0.5 — — — — — — — — — — — — — 3 — 40 7.3 1.9 0.5 — — — — — — — — — —— — — 4 — 55 12 2.7 0.7 — — — — — — — — — — — — — 5 — — 18 4.2 1.2 — — —— — — — — — — — — — 6 — — 19 4.7 1.4 — — — — — — — — — — — — — 7 — — 286.5 1.8 0.5 — — — — — — — — — — — — 8 — — — 16 4.0 1.1 — — — — — — — — —— — — 9 — — — 17 4.5 1.2 — — — — — — — — — — — — 10 — — — 21 5.7 1.5 0.5— — — — — — — — — — — 11 — — — 31 8.8 2.3 0.8 — — — — — — — — — — — 12 —— — 42 12 3.2 1.0 — — — — — — — — — — — 13 — — — 42 12 3.2 1.0 — — — — —— — — — — — 14 — — — 42 12 3.5 1.1 — — — — — — — — — — — 15 — — — 68 206.0 2.1 — — — — — — — — — — — 16 — — — — 37 10 3.3 1.0 — — — — — — — — —— 17 — — — — 48 13 3.8 1.2 — — — — — — — — — — 18 — — — — — 13 4.0 1.3 —— — — — — — — — — 19 — — — — — 16 4.5 1.3 — — — — — — — — — — 20 — — — —— 23 6.7 1.8 0.6 — — — — — — — — — 21 — — — — — — 13 4.2 1.6 0.6 — — — —— — — — 22 — — — — — — — 18 5.8 1.7 — — — — — — — — 23 — — — — — — — 216.9 2.2 — — — — — — — — 24 — — — — — — — 23 7.3 2.3 0.7 — — — — — — — 25— — — — — — — 33 10 2.8 0.8 — — — — — — — 26 — — — — — — — — 10 2.5 0.7— — — — — — — 27 — — — — — — — — 10 3.7 1.3 — — — — — — — 28 — — — — — —— — 13 3.7 1.1 — — — — — — — 29 — — — — — — — — 15 3.9 1.3 — — — — — — —30 — — — — — — — — 18 5.3 1.6 0.5 — — — — — — 31 — — — — — — — — 18 5.41.5 0.5 — — — — — — 32 — — — — — — — — 30 10 3 1.1 — — — — — — 33 — — —— — — — — — 19 5.6 2.2 0.7 — — — — — 34 — — — — — — — — — 21 5.7 1.8 0.6— — — — — 35 — — — — — — — — — 25 7.6 2.3 0.8 — — — — — 36 — — — — — — —— — 28 7.9 3.4 1.3 — — — — — 37 — — — — — — — — — 32 10 3.3 1.2 — — — —— 38 — — — — — — — — — 32 10 3.3 1.2 — — — — — 39 — — — — — — — — — — 186.3 2.1 0.7 — — — — 40 — — — — — — — — — — 24 8.2 2.8 0.9 — — — — 41 — —— — — — — — — — 18 6.0 2.3 0.9 — — — 42 — — — — — — — — — — — — — — 3013 5.3 — 43 — — — — — — — — — — — — — — — 33 13 5.3 44 — — — — — — — — —— — — — — — — — 75 ^(*))EHPC Di(2-ethylhexyl) peroxydicarbonate CEPCDicetyl peroxydicarbonate MYPC Dimyristyl peroxydicarbonate CHPCDicyclohexyl peroxydicarbonate BCHPC Di(4-tert-butylcyclohexyl)peroxydicarbonate ¹⁾0.1 molar in toluene ²⁾0.1 molar in isododecane³⁾0.1 molar in mineral oil ⁴⁾0.1 molar in styrene ⁵⁾extrapolatedrecommended values

TABLE 2 INTEROX No. Code Chemical name 1 ASCP ACETYL CYCLOHEXANESULFONYL PEROXIDE 2 DIPND 1,4-DI(2-NEODECANOYLPEROXYISOPROPYL)BENZENE 3CUPND CUMYLPEROXY NEODECANOATE 4 PEROXYDICARBONATE 5 IPPCDIISOPROPYLPEROXY DICARBONATE 6 TAPND Tert-AMYLPEROXY NEODECANOATE 7TBPND Tert-BUTYLPEROXY NEODECANOATE 8 TAPPI Tert-AMYLPEROXY PIVALATE 9DCLBP DI(2,4-DICHLOROBENZOYL) PEROXIDE 10 TBPPI Tert-BUTYLPEROXYPIVALATE 11 INP DI(3,5,5-TRIMETHYLHEXANOYL) PEROXIDE 12 DP DIDECANOYLPEROXIDE 13 LP DILAUROYL PEROXIDE 14 DI(2-METHYLBENZOYL) PEROXIDE 15DHPEH 2,5-DIMETHYL-2,5-DI(2-ETHYLHEXANOYLPEROXY)HEXANE 16 PMBPDI(4-METHYLBENZOYL) PEROXIDE 17 BP DIBENZOYL PEROXIDE 18 TAPEHTert-AMYLPEROXY 2-ETHYLHEXANOATE 19 TBPEH Tert-BUTYLPEROXY2-ETHYLHEXANOATE 20 Tert-BUTYLPEROXY ISOBUTYRATE 21 TBPMTert-BUTYLMONOPEROXY MALEATE 22 TMCH1,1-DI(tert-BUTYLPEROXY)-3,3,5-TRIMETHYLCYCLOHEXANE 23 CH1,1-DI(tert-BUTYLPEROXY)CYCLOHEXANE 24 Tert-BUTYLPEROXYISOPROPYLCARBONATE 25 TBPIN Tert-BUTYLPEROXY 3,5,5-TRIMETHYLHEXANOATE 26DHPBZ 2,5-DIMETHYL-2,5-DI(BENZOYLPEROXY)HEXANE 27 TBP-EHCTert-BUTYLPEROXY (2-ETHYLHEXYL)CARBONATE 28 Tert-BUTYLPEROXY ACETATE 29TABP Tert-AMYLPEROXY BENZOATE 30 TBPB Tert-BUTYLPEROXY BENZOATE 31 BU2,2-DI(tert-BUTYLPEROXY)BUTANE 32 NBV n-BUTYL-4,4-DI(tert-BUTYLPEROXY)VALERATE 33 EBU ETHYL-3,3-DI(tert-BUTYLPEROXY) BUTYRATE 34 DCUP DICUMYLPEROXIDE 35 BCUP Tert-BUTYLCUMYL PEROXIDE 36 DTAP DI(tert-AMYL) PEROXIDE37 DIPP DI(tert-BUTYLPEROXYISOPROPYL)BENZENE 38 DHBP2,5-DIMETHYL-2,5-DI(tert-BUTYLPEROXY)HEXANE 39 DTBP DI(tert-BUTYL)PEROXIDE 40 DYBP 2,5-DIMETHYL-2,5-DI(tert-BUTYLPEROXY)HEX-3-YNE 41 HMCN3,3,6,6,9,9-HEXAMETHYL-1,2,4,5-TETRAOXACYCLONONANE 42 TBHP Tert-BUTYLHYDROPEROXIDE 43 CC DFH 3,4-DIMETHYL-3,4-DIPHENYLHEXANE 44 CC DFB2,3-DIMETHYL-2,3-DIPHENYLBUTANE

TABLE 3 Chemical Name Structural Formula V-70 2,2′-Azobis(4-methoxy-2,4-dimethylvaleronitrile)

M.W. 308.42 V-65 2,2′-Azobis(2,4-dimethyl- valeronitrile)

M.W. 248.37 V-60 2,2′-Azobisisobutyronitrile

M.W. 164.21 V-601 Dimethyl 2,2′-azobisisobutyrate

M.W. 230.26 V-59 2,2′-Azobis(2-methyl- butyronitrile)

M.W. 192.26 V-40 1,1′-Azobis(1-cyclo- hexanecarbonitrile)

M.W. 244.34 V-30 2-(Carbamoylazo)- isobutyronitrile

M.W. 140.14 VR-110 2,2′-Azobis(2,4,4-trimethyl- pentane)

M.W. 254.46 V-19 2-Phenylazo-2,4-dimethyl- 4-methoxyvaleronitrile

M.W. 245.32 VR-160 2,2′-Azobis(2-methylpropane)

M.W. 142.24 VA-044 2,2′-Azobis (N,N′-dimeth- yleneisobutyramidine)dihydrochloride

M.W. 323.27 V-50 2,2′-Azobis (2-amidino- propane) dihydrochloride

M.W. 271.19 VA-061 2,2′-Azobis (N,N′-dimeth- yleneisobutyramidine)

M.W. 250.35 V-501 4,4′-Azobis (4-cyano- pentanoic acid)

M.W. 280.28 VA-080 2,2′-Azobis{2-methyl-N- [1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}

M.W. 408.45 VA-082 2,2′-Azobis{2-methyl-N- [1,1-bis(hydroxymethyl)ethyl] propionamide}

M.W. 376.45 VA-086 2,2′-Azobis [2-methyl-N- (2-hydroxyethyl)propionamide]

M.W. 288.35 VA-088 2,2′-Azobis(isobutyramide) dihydrate

M.W. 236.27 Wako Pure Chemical Industries, Ltd. 10 hour half-lifedecomposition Solubility at 20° C. (g/100 g solvent) Appearance MeltingRange temperature Toluene n-Hexane Ethanol Methanol Water V-70 Whitecrystalline 50-96° C. 30° C. in toluene 3.3 (Benzene) 1 at 25° C. — 2.1at 25° C.  powder (Decomposition) V-65 White crystals 45-70° C. 51° C.in toluene 72 4 20.5 at 25° C. 22  V-60 White crystalline 100-103° C.65° C. in toluene 7 at 25° C. — 3 at 25° C. 7.5 at 25° C.  powder V-601Pale yellow wax 22-28° C. 66° C. in toluene ◯ ◯ ◯ ◯  (Decompositionpoint 85-87° C.) V-59 White crystalline 55-57° C. 67° C. in toluene ◯ —◯ ◯  powder (Decomposition point 84-87° C.) V-40 White crystals113-115° C. 88° C. in toluene 33 (Benzene) 0.9 — 3.2  V-30 Pale yellowcrystals 76-78° C. 104° C. in toluene — — ◯ ◯ ◯ VR-110 Pale yellowcrystals 23-24° C. 110° C. in ◯ ◯ ◯ —  or pale yellow liquiddiphenylether V-19 Transparent (Boiling point 122° C. in xylene ◯ — ◯ ◯ yellow liquid 149-150° C./1 mmHg) VR-160 Pale yellow liquid (Boilingpoint 160° C. gas phase ◯ ◯ ◯ ◯  109-110° C.) VA-044 White to pale188-193° C. 44° C. in water   0.02 1.6 35.2 yellow crystals(Decomposition) or crystalline powder V-50 White or off-white 160-169°C. 56° C. in water   — 2.1 23.2 granular (Decomposition) VA-061 Paleyellow 115-125° C. 61° C. in methanol  — 2.8 9.4 0.5 powder(Decomposition) V-501 White crystalline 120-123° C. 69° C. in water  —◯ — 1 powder (Decomposition) VA-080 Pale yellow 150-155° C. 80° C. inwater   — 1.8 2.0 crystalline powder (Decomposition) VA-082 Paleyellow 156-161° C. 82° C. in water   — 5.1 0.6 crystalline powder(Decomposition) VA-086 Pale yellow 140-145° C. 86° C. in water   — 4.52.4 crystalline powder (Decomposition) VA-088 Pale yellow 88-92° C. 88°C. in water   3.6 9.6 0.8 crystalline powder (Decomposition) ◯ Readilysoluble  Insoluble

What is claimed is:
 1. A method for producing a block-shaped orplate-shaped polymethacrylimide foamed material, comprising:copolymerizing a copolymerization mixture comprising methacrylic acid,methacrylonitrile and optionally a copolymerizable monomer in thepresence of a radical-forming initiator to obtain a copolymer;post-polymerizing and cyclizing said copolymer to polymethacrylimide;and foaming said polymethacrylimide to obtain a foamed material; whereinsaid copolymerizing is performed in the presence of a mixture comprisingat least four initiators with graduated half lives; wherein each of theinitiators has a half live of ½ hour at a certain temperature and saidtemperature differs by at least 10° C. for each of said initiators. 2.The method according to claim 1, wherein a mixture of slow, middle andfast decomposing agents having half lives of one half hour in the rangesof from 40 to 60° C., 60 to 85° C., 85 to 110° C. and 110 to 130° C. isused as initiator mixture.
 3. The method according to claim 1, wherein amixture of slow, middle and fast decomposing agents having half lives ofone half hour in the ranges of from 50 to 57° C., 70 to 80° C., 85 to105° C. and 115 to 125° C. is used as initiator mixture.
 4. The methodaccording to claim 1, wherein said copolymerizing is tempered completelyor in stages using a water bath, whose temperature is constantthroughout an entire polymerization time or for each stage.
 5. Themethod according to claim 1, wherein the copolymerization mixturecomprises a filler.
 6. The method according to claim 5, wherein thecopolymerization mixture comprises an electrically conductive particle.7. A foamed block or a foamed plate of polymethacrylimide, obtained bythe method according to claim
 1. 8. The foamed block or foamed plateaccording to claim 7, having a thickness of from 80 to 300 mm.
 9. Amethod for producing a polymer plate, comprising: copolymerizingmethacrylic acid, methacrylonitrile and optionally a copolymerizablemonomer in the presence of radical-forming initiators to obtain acopolymer; and post-polymerizing said copolymer; wherein saidcopolymerizing is performed in the presence of a mixture comprising atleast four initiators with graduated half lives; wherein each of theinitiators has a half live of ½ hour at a certain temperature and saidtemperature differs by at least 10° C. for each of said initiators. 10.A polymer plate obtained by the method according to claim
 9. 11. Amethod, comprising: constructing an aircraft comprising the foamed blockor the foamed plate according to claim
 7. 12. A method, comprising:constructing an aircraft comprising the foamed block or the foamed plateaccording to claim
 8. 13. The method according to claim 6, wherein saidelectrically conductive particle comprises a carbon fiber.
 14. Themethod according to claim 1, wherein said copolymerizing proceeds underisothermal conditions.